The present invention relates to a vehicle steering system including electrohydraulic controls for automatically energizing an electric motor for driving a backup pump for supplying power steering fluid upon the failure of a main pump to output the necessary fluid, and more specifically relates to an improvement of the control system disclosed in U.S. Pat. No. 3,820,620 issued to Miller et al on June 28, 1974.
The Miller et al patent discloses both an open center steering system, wherein a fixed displacement main pump is used together with a flow sensing switch for monitoring the output thereof, and a closed center steering system, wherein a variable displacement main pump is used together with a pressure sensing switch for monitoring the output thereof. The present invention deals only with the closed center version of the steering system.
It has been found that with a closed center hydraulic steering system that both pressure and flow can vary from almost zero to maximum in various combinations during normal machine operations. Consequently, the patented system has the drawback that under some conditions of normal operation the pressure may fall below that at which the pressure switch is set thus resulting in unnecessary actuation of the electric motor for driving the backup fluid supply pump.
Another drawback of the patented system is that once the primary system has failed and the backup system has become actuated, the electric motor runs continuously even though a continuous requirement for power steering fluid is not present thus resulting in the battery power being continuously drained in the case where the main steering fluid supply pump is not being driven due to engine failure.